1. Field of the Invention
This invention relates to an exhaust gas recirculation (EGR system) in which an exhaust gas recirculation passage (EGR passage) extending between an intake system and an exhaust system in an internal combustion engine is opened or closed by an exhaust gas recirculation valve (EGR valve) to selectively recirculate exhaust gas to the intake system, and especially to a method and system for the detection of a fault of such an EGR system.
2. Description of the Related Art
As a conventional fault detection system for an EGR system, there has been proposed such a system that an EGR valve is opened or closed in an EGR operation zone but if the difference between a value detected on the volume of air inducted before the opening or closure and that detected after the opening or closure falls within a predetermined range, an alarm is given [see Japanese Patent Application Laid-Open (Kokai) No. SHO 62-51747].
Such a conventional fault detection system for an EGR system however performs a self-diagnosis even when the pressure difference between a pressure within said throttle valve downstream side intake passage and a pressure within said throttle valve upstream side intake passage exceeds a critical pressure as in an operation zone in which the throttle valve is controlled at a low angle. The term "critical pressure" as used herein means a pressure at which the flow velocity of inducted air flowing past the throttle valve reaches the velocity of sound, for example, a pressure of 420-430 mmHg on the downstream side of the throttle valve when the pressure on the upstream side of the throttle valve, namely, the atmospheric pressure is 760 mmHg. Paying attention to the difference between a value detected on the volume of air inducted before the opening or closure of the EGR valve and that detected after the opening or closure, this difference falls within the predetermined range in the above case even if the EGR system is in order. The conventional fault detection system therefore involves the potential problem that the EGR system is erroneously determined to be out of order, resulting in production of an alarm.